When mounting a progressive lens in a frame, it is important for the visual comfort of the user to ensure that the lens is properly positioned relative to the eye for which it corrects a refractive or accommodation defect.
The ophthalmic lens is centered when the reference center of the lens specified in its design and the center of the pupil of the eye are in alignment, or in other words, when the line of regard passes through the reference center of the lens. Centering is thus the result of bringing together two geometrico-optical givens: the morphology of the wearer's pupil and the position on the lens of the reference center.
During fabrication, every progressive lens is provided with temporary markings using paint, and with permanent markings using engraving. The temporary markings enable the lens to be centered conveniently prior to being mounted. The permanent markings make it possible on the patient's frame to identify the nature of the progressive ophthalmic lens, the value of its addition, and also to verify or reestablish the exact indexing of said lens, even after the temporary markings have been removed. It should be understood that the temporary marks are removed by the optician before handing the spectacles over to a customer and that, where necessary, they can be reestablished on the basis of the permanent engraved markings that remain on the lens.
More precisely, as shown in FIG. 10, the temporary markings conventionally comprise:                a “fitting” or “centering” cross 11 identifying the center of the far vision zone that is to be positioned at the center of the wearer's pupil when the wearer is looking straight ahead at infinity; it serves to position the power progression of the lens 10 vertically relative to the eye so that, as intended by the designer of the lens, the wearer can easily find the correcting powers needed for far vision, intermediate vision, and near vision;        a central point 12 locating the “optical center” of the lens 10 and situated 2 millimeters (mm) to 6 mm below the mounting cross 11, depending on the type of the lens; for a progressive lens, this “optical center” is conventionally the “prism reference” point where the nominal prismatic power of the lens 10 corresponding to the wearer's prescription is measured;        a circle 13 for measuring the far vision power of the lens situated in the upper portion of the lens 10 immediately above the fitting cross 11 and locating the reference point for far vision; this is thus the place where a frontofocometer should be placed in order to measure the far vision power of the lens 10;        a circle 14 for measuring the near vision power of the lens and situated in the bottom portion of the lens 10, surrounding the center or reference point of the near vision zone; this center is offset towards the nose by 2 mm to 3 mm, and the distance between it and the fitting cross 11 constitutes the nominal length of the progression of the lens 10; and        one or more lines 15 identifying the horizontal direction for the lens 10 and for use in centering.        
As can also be seen in FIG. 10, the permanent markings generally comprise:                two small circles or signs 16 located on the horizontal line of the lens 10 passing through its optical center and always situated 17 mm on either side of the optical center 12; these engravings serve to find the horizontal and vertical centering of the lens;        a sign 17 serving to identify the trademark and the exact nature of the progressive lens (e.g. V for Varilux®) which is engraved under the nasal small circle or sign; and        a two- or three-digit number representing the value of the addition (e.g. 30 or 300 for an addition of 3.00 D) which is engraved beneath the temporal small circle or sign.        
It should be recalled that for multiple focus lenses presenting one or more lines of power discontinuity, said lines act as permanent markings.
The centering of a progressive lens comprises two components: a vertical component and a horizontal component.
Vertical centering enables the power progression of the lens to be positioned vertically in front of the eye in such a manner as to make it easy for the eye to find the needed correcting power in the manner intended by the designer of the lens. The far vision correction power should be reached on the axis of the primary position of regard and the near vision power on the axis of regard when lowered for near vision.
This vertical centering is conventionally performed by using the far vision centering cross that is painted for this purpose on the lens by the manufacturer: centering consists in positioning the centering cross of the lens in front of the center of the pupil of the patient's eye when looking at infinity. In practice, the optician measures the height between the bottom of the frame and the center of the wearer's pupil when looking horizontally, and positions the centering cross of the lens at the measured height.
Horizontal centering consists in positioning the progressive lens laterally relative to the eye in such a manner that the wearer can make optimum use of the zones for far vision, intermediate vision, and near vision.
Physiological studies have shown that in 25% of cases the centers of the pupils present horizontal asymmetry of more than 1 mm relative to the nose, and in 60% of cases a virtual offset of more than 1 mm. It is therefore advantageous to be able to verify the centering of each of the two lenses independently of the other, and that is why it is preferable to be able to measure the right and left half-distances between the pupils rather than measuring only the overall interpupillary distance.
By construction, all progressive lenses possess relative positioning of the far vision and near vision zones with the near vision zone being off-centered towards the nose relative to the far vision zone.
The horizontal centering of the lens can thus be undertaken either relative to near vision or relative to far vision (the more usual technique).
Centering relative to far vision consists in measuring the half-distances between the right and left pupils of the patient with a far vision regard, i.e. the distances between the root of the nose and the centers of the pupils (or more precisely of the corneal reflections) of the right eye and of the left eye. The far vision fitting crosses of the right and left lenses are then positioned at these distances from the mid or nasal plane of the frame.
With the lens appropriately oriented about its optical or central axis, with its horizontal lines (or engraved circles) in alignment on the horizontal of the frame, the near vision zone is found, by construction of the lens, to be appropriately off-centered towards the nose by 2 mm to 3 mm relative to the far vision zone.
Centering relative to near vision is performed more rarely, and is undertaken in analogous manner by measuring the half-distances between the pupils of the patient with near vision regard and by positioning the centers of the near vision zones of the left and right lenses at said distances.
This second technique presents a particular advantage when the right and left eyes converge asymmetrically.